Not only do communities have a life cycle, people's membership in communities does. People give all sorts of reasons for leaving a community (e.g. boredom, other interests, deciding the community is full of assholes, an incident they write over one megabyte of text complaining about), but the length of participation is typically 12 to 18 months regardless. Anything over that, you're a previous generation.

So I wouldn't be disappointed unless they stopped before 12-18 months.